1. Field of the Invention
The present invention relates to a display, such as a liquid crystal display (LCD) panel and more particularly to a display device and a driving method thereof that speeds up optical response and is suitable for displaying continuous images with rapid motions.
2. Description of the Related Art
Progress has been ceaselessly made in the manufacturing technique for liquid crystal displays regarding high contrast displays with a wide view angle. However, continuous images with rapid motions are displayed at the expense of blurred images, as an image change takes place later than variations in motions do. So far various related driving techniques have been put forth in an attempt to shorten the time liquid crystal displays take to respond. Of these, the capacitively coupled driving (CCD) method puts forth by the Japanese Matsushita Electric Industrial Co., Ltd. shortens the time pixel electrodes take to respond to variations in voltage best, and in consequence it speeds up changes in the electric field of a liquid crystal capacitor.
FIG. 1 is an equivalent circuit diagram of a conventional liquid crystal display. The liquid crystal display 10 is composed of a plurality of parallel data lines 121-12n and a plurality of scanning lines 111-11m disposed perpendicular to the data lines, which further comprises a plurality of pixels 13 are placed at the encircled areas of the data lines 121-12n and the scanning lines 111-11m. Each pixel 13 comprises a thin film transistor 131 and a liquid crystal capacitor 133, which control the direction in which liquid crystal molecules tilt. For instance, the thin film transistor 131 is controlled by a pulse Φ2 of the scanning line 112 to be turned on or turned off, whereas the two electrodes of the liquid crystal capacitor 133 are connected to a pixel electrode 134 and a common electrode 135, respectively. In addition, each of the pixels 13 comprises a storage capacitor 132 whose two electrodes are connected to the pixel electrode 134 and the scanning 111, respectively. With the storage capacitor 132, the operating voltage of the pixel electrode 134 is kept within a preferred voltage range so as to reduce the leakage resulting from the properties of liquid crystal materials and other stray capacitance.
Furthermore, a coupled voltage is induced on the pixel electrode 134 by employing the scanning signals Φ1-Φm, with four gate voltage levels. Given the coupled voltage, the electric field of the liquid crystal capacitor 133 varies faster. However, real changes of gray levels always occur later than variations in the electric field, as liquid crystal molecules tilt slower than the electric field varies.
FIG. 2(a) is a conventional waveform diagram of the optical responses of pixels and data signals. The waveforms of the driving voltage applied to a pixel electrode are shown in the lower half of the figure. In the upper half of the figure, the dotted line indicates the theoretical optical responses of pixels, whereas the solid line indicates the actual optical responses of pixels. If a second default voltage V1 applied to the pixel electrode changes into a first default voltage V2, the actual positions or states of the liquid crystal molecules vary and thus the transmittance of rays from a backlight source decreases, compared with the ideal state, which is a true white display without any applied voltage. It takes about two vertical scanning periods to pass through a transient time where transmittance decreases from L2 to L1, thus the conventional technology is unfit for displaying continuous images with rapid motions.
The concept of fast response driving is put forth to speed up optical responses, as shown in FIG. 2(b). The default voltages V2 and V1 applied to the pixel electrode are replaced with V2′ and V1′, where V2<V2′ and |V1|<|V1′|. Hence, the time taken to pass through a transient time in which transmittance decreases from L2 to L1 can be reduced to approximately one vertical scanning period, indicating that the fast response driving method surpasses the driving method described in FIG. 2(a) in displaying continuous images with rapid motions. Furthermore, the deviation area A′ (that is, the hatched area enclosed by the solid line and the dotted line) of the fast response driving method is less than the hatched area A in FIG. 2(a), thus the fast response driving method seldom brings about blurred motion images.
FIG. 2(c) is a diagram about the waveforms of the optical responses resulting from the Dynamic Contrast Compensating Driving method put forth by Japanese Hitachi Ltd. Wherein the driving voltages V2″>V2 and |V1″|>|V1|, an actual optical response ends up with an overshooting waveform during a vertical scanning period and results in the return of the default transmittance L2 or L1 during the following period. The overshooting-related area B is roughly equal to the area A″ so as to compensate for a lack of dynamic contrast in motion images. Nevertheless, it still takes longer time than one vertical scanning period to bring about an overshooting waveform with the dynamic contrast compensating driving method, making it impossible to apply the dynamic contrast compensating driving method to motion images, which take less than 16.7 ms to give an optical response.